1. Field of the Invention
The present invention relates to an optical pickup apparatus for reading and recording information on recording media (hereinafter referred to as “optical pickup apparatus”), more particularly to an optical pick apparatus in which an actuator is made lightweight, whereby high-speed operation and cost reduction are realized.
2. Description of the Related Art
Currently, in an optical pickup apparatus using light, such as CD (compact disk) drive, information is read out in such a manner that a recording pit is produced by converging a laser beam emitted from a laser beam source, as a micro spot, onto a track provided on a disk-like recording medium such as a CD, presence or absence of the pit is recorded as information, the micro spot is radiated on the track, and that the presence or absence of the pit on the track is detected by a reflected laser beam.
Recently, DVDs (digital video disks) having a recording capacity about 7 times as large as that of CDs have been widely used to meet the demand of increased recording capacity. Increase in recording capacity means improvement of the recording density, which depends on the number of recording pits formed on a recording medium (hereinafter referred to as “disk”). In DVDs, one key way for increasing the recording density is decreasing the size of a recording pit, that is, decreasing the diameter of a laser beam spot on the disk. The size of the micro spot to be radiated on the disk is proportional to the wavelength of a laser beam and is inversely proportional to the numerical aperture of an objective lens. Accordingly, for decreasing the size of the recording pit, it is required to shorten the wavelength of the laser beam and/or to increase the numerical aperture of the objective lens.
However, DVDs are strongly required to be compatible with CDs for inheriting software resources. Originally, an optical pickup apparatus was provided with one laser beam source for a laser beam with a wavelength of 635 to 650 nm and one objective lens having a numerical aperture of about 0.6 for DVDs and also with another laser beam source for another laser beam with a wavelength of 780 nm and another objective lens having a numerical aperture of about 0.45 for CDs so as to ensure the compatibility between both disks.
However, when the numerical aperture of the objective lens is increased for DVDs, the convergence state of the laser beam deteriorates due to increased coma aberration with respect to the inclination of the optical disk. Since coma aberration is proportional to the thickness of the disk substrate as well as to the third power of the numerical aperture of the objective lens, the DVDs are designed to have a disk substrate thickness of 0.6 mm, which is half that of the CDs.
When the thickness of the disk substrate deviates from the prescribed value, spherical aberration occurs at a convergence position of light passing through the inward portion of the objective lens and a convergence position of light passing through the outward portion. Therefore, when a CD is read by using an objective lens with a numerical aperture of 0.6 optimized to the thickness of the DVD substrate, the spherical aberration must be corrected by limiting the outward portion of luminous flux incident on the lens or by slightly diverging the incident angle at the lens.
Thus, one objective lens adapted for the DVD may work compatibly for the CD with the necessary correction of spherical aberration, but two laser beam sources for laser beams having the above-mentioned respective different wavelengths have to be provided for a write-once-read-many CD because the reflective recording layer of the CD is formed of an organic dye material and has a reflection coefficient as low as 6% for a laser beam having a wavelength of 635 to 650 nm appropriate to the DVD.
The present inventors disclosed “Optical Pickup Apparatus for Reading and Recording Information on Recording Medium” in Japanese Patent Application No. 2000-401788. The optical pickup apparatus includes one semiconductor laser and a super-resolution cutoff filter disposed before an objective lens and reads and records information on two kinds of recording media each having a different recording density from other.
The super-resolution cutoff filter is an optical filter in coherent light using a technology called super-resolution. Since the super-resolution is a well-known technology and is described in detail in Optical and Electro-Optical Engineering Contact Vol. 33, No. 11 (1995) and the like, a description thereof will be omitted.
The resolving power of an optical device is related to the diameter of a laser beam spot converged by an objective lens. It is well known that a formula of W=1.22λ/NA is valid, where W is the diameter of the main lobe of a convergent spot in paraxial approximation, λ is the wavelength of a laser beam, and NA is the numerical aperture. Accordingly, the wavelength λ has to be small and the numerical aperture NA has to be large in order to decrease the diameter W of the main lobe, that is, to obtain high resolution.
FIGS. 7A to 7D explain the relation between the configuration of the super-resolution cutoff filter and the distribution of the diameter of the main lobe of a converged laser beam spot, in which FIGS. 7A and 7C show the configurations of the super-resolution cutoff filters, and FIGS. 7B and 7D show the distribution of the diameters of the main lobes obtained by respective configurations. The diameter W of the main lobe is called a diffraction-limited spot diameter, is normally the minimum spot diameter realizable, and shows the distribution as shown in FIG. 7B when a laser beam 50 is incident on an objective lens 16 shown in FIG. 7A. However, when a light-blocking plate 40 is placed right before the objective lens 16 whereby the distribution of the amplitude in the lens aperture surface is transformed so as to be small at the center and large at the outside periphery, the diameter of the main lobe of the converged spot can be smaller than the normal diffraction-limited value thereby obtaining the distribution shown in FIG. 7D.
FIG. 8 shows an optical pickup apparatus disclosed in the aforementioned Japanese Patent Application No. 2000-401788. The optical pickup apparatus includes a semiconductor laser 12 for a laser beam with a wavelength of 780 nm for a CD, a half mirror 11 which is a beam splitter element for reflecting and guiding the laser beam to a collimating lens 13 and also for transmitting and guiding a reflected laser beam from a disk 18 to a photo-detector 90 adapted for the wavelength of the semiconductor laser for a CD, and a reflecting mirror 15 for guiding the laser beam having passed through the collimating lens 13 to an objective lens 16 or 17 for making the laser beam converged and incident on the disk 18. The disk 18, either a DVD 18a or a CD 18b, is placed on a driving mechanism (not shown) and rotated thereby.
The objective lens 16 has a high numerical aperture (high NA) for a DVD. The objective lens 17 has a low numerical aperture (low NA) for a CD. The laser beam having passed through the collimating lens 13 is incident on the objective lens 16 or 17 selected according to the DVD 18a or the CD 18b. A super-resolution cutoff filter 10 is placed right before the objective lens 16. When reading the DVD 18a, the objective lens 16 combined with the super-resolution cutoff filter 10 as a unit is arranged at an optical path, whereas when reading the CD 18b, the objective lens 17 alone is arranged at the optical path. The objective lens 16 combined with the super-resolution cutoff filter 10, and the objective lens 17 are integrated with a device for switching over the lenses and an actuator driving system for controlling their position relative to a recording surface of the disk, and are interchanged for the DVD and the CD by a driving mechanism (not shown).
The laser beam reflected from the disk 18 starts traveling in the backward direction along the incoming path, passes through the half mirror 11, is directed to the photo-detector 90, and converted thereby into an electrical signal.
The super-resolution cutoff filter 10 comprises a transparent plate glass 33 and two anti-reflection films 31 each made of a multi-layer and formed on the both side surfaces of the transparent plate glass 33, respectively, such that one is formed entirely on one side surface and the other is formed partly on the other side surface, specifically, formed at portions except a central portion provided with a metal reflection film 32.
The optical pickup apparatus in FIG. 8 operates as follows. When reading and recording on the DVD 18a, a laser beam emitted from the semiconductor laser (a wavelength of 780 nm) 12 for a CD is reflected by the half mirror 11 so as to be directed to the collimating lens 13. The laser beam has its divergence angle collimated by the collimating lens 13 to get into a parallel pencil, has its optical path changed by the reflecting mirror 15, passes through the super-resolution cutoff filter 10, is converged by the objective lens 16 so as to have a predetermined spot diameter, and incident on a recording surface of the DVD 18a. In this instance, the objective lens 16 combined with the super-resolution cutoff filter 10 for the DVD is selected in advance and set in place by the driving mechanism (not shown) to be ready for operation.
The laser beam reflected at the recording surface of the DVD 18a starts traveling in the backward direction along the incoming path, passes through the objective lens 16 and the super-resolution cutoff filter 10, has its optical path changed by the reflecting mirror 15, is converged by the collimating lens 13, passes through the half mirror 11, is incident on the photo-detector 90, and converted thereby into an electrical signal. When a signal is recorded on the DVD 18a, the intensity of the laser bream is increased to a predetermined value, and when recorded information is read, the intensity of the laser bream is decreased to a predetermined value. This is controlled by a control circuit and a laser driving circuit (both circuits not shown).
When reading the CD 18b, only the objective lens 17 for the CD is selected in advance and set in place by the driving mechanism (not shown) to be ready for operation. Also, when information recorded on the CD is read, the intensity of the laser beam is controlled to be a predetermined value by the control circuit and the laser driving circuit (both circuits not shown).
However, the optical pickup apparatus shown in FIG. 8 has the following disadvantage. Though the super-resolution cutoff filter 10 placed right before the objective lens 16 enables the information to be compatibly read and recorded on two kinds of disks having respective different recording densities with only one semiconductor laser for a laser beam with a wavelength of 780 nm for a CD, two objective lenses, specifically, one having a high numerical aperture (high NA) for DVDs and another having a low numerical aperture (low NA) for CDs, are required to make the apparatus work compatibly for the both disks, and the two objective lenses are mounted on the same driving mechanism, and are selected and switched over thereby together with the super-resolution cutoff filter, prohibiting reduction in the weight. This prevents the optical pickup apparatus from operating at a high-speed and being produced inexpensively.